Bone is a dynamic connective tissue comprised of functionally distinct cell populations required to support the structural, mechanical and biochemical integrity of bone and the human body's mineral homeostasis. The principal cell types involved include, osteoblasts responsible for bone formation and maintaining bone mass, and osteoclasts responsible for bone resorption. Osteoblasts and osteoclasts function in a dynamic process termed bone remodelling. The development and proliferation of these cells from their progenitors is governed by networks of growth factors and cytokines produced in the bone microenvironment as well as by systemic hormones. Bone remodelling is ongoing throughout the lifetime of the individual and is necessary for the maintenance of healthy bone tissue and mineral homeostasis. The process remains largely in equilibrium and is governed by a complex interplay of systemic hormones, peptides and downstream signalling pathway proteins, local transcription factors, cytokines, growth factors and matrix remodelling genes.
Any interference or imbalance arising in the bone remodelling process can produce skeletal disease, with the most common skeletal disorders characterized by a net decrease in bone mass. A primary cause of this reduction in bone mass is an increase in osteoclast number and/or activity. The most common of such disease, and perhaps the best known, is osteoporosis occurring particularly in women after the onset of menopause. In fact osteoporosis is the most significant underlying cause of skeletal fractures in late middle-aged and elderly women. While estrogen deficiency has been strongly implicated as a factor in postmenopausal osteoporosis, there is longstanding evidence that remodelling is a locally controlled process being that it takes place in discrete packets throughout the skeleton as first described by Frost over forty years ago (Frost H. M. 1964).
Since bone remodelling takes place in discrete packets, locally produced hormones and enzymes may be more important than systemic hormones for the initiation of bone resorption and the normal remodelling process. Such local control is mediated by osteoblasts and osteoclasts in the microenvironment in which they operate. For example, osteoclasts attach to the bone matrix and form a separate compartment between themselves and the bone surface delimited by a sealing zone formed by a ring of actin surrounding the ruffled border. Multiple small vesicles transport enzymes toward the bone matrix and internalize partially digested bone matrix. The microenvironment within the sealing zone is rich with the presence of lysosomal enzymes and is highly acidic compared to the normal physiological pH of the body. The ruffled border membrane also expresses RANK, the receptor for RANKL, and macrophage-colony stimulating factor (M-CSF) receptor, both of which are responsible for osteoclast differentiation, as well as the calcitonin receptor capable of rapidly inactivating the osteoclast (Baron, R. 2003).
In a complex pattern of inhibition and stimulation, growth hormone, insulin-like growth factor-1, the sex steroids, thyroid hormone, calciotrophic hormones such as PTH and prostaglandin E2, various cytokines, such as interleukin-1 beta, interleukin-6, and tumor necrosis factor-alpha, and 1,25-dihydroxyvitamin D (calcitriol) act co-ordinately in the bone remodelling process (Jilka et al. 1992; Poli et al. 1994; Srivastava et al. 1998; de Vemejoul 1996).
Thus, it stands to reason that the unique local environments created by these specialized cells is due to the expression of either unique genetic sequences not expressed in other tissues and/or splice variants of polynucleotides and polypeptides expressed in other tissues. The isolation and identification of polynucleotides, polypeptides and their variants and derivatives specific to osteoclast activity will permit a clearer understanding of the remodelling process and offer tissue specific therapeutic targets for the treatment of disease states related to bone remodelling.
Many diseases linked to bone remodelling are poorly understood, generally untreatable or treatable only to a limited extent. For example, osteoarthritis is difficult to treat as there is no cure and treatment focuses on relieving pain and preventing the affected joint from becoming deformed. Non-steroidal anti-inflammatory drugs (NSAIDs) are generally used to relieve pain.
Another example is osteoporosis where the only current medications approved by the FDA for use in the United States are the anti-resorptive agents that prevent bone breakdown. Estrogen replacement therapy is one example of an anti-resorptive agent. Others include alendronate (Fosamax—a biphosphonate anti-resorptive), risedronate (Actonel—a bisphosphonate anti-resorptive), raloxifene (Evista—selective estrogen receptor modulator (SERM)), calcitonin (Calcimar—a hormone), and parathyroid hormone/teriparatide (Forteo—a synthetic version of the human hormone, parathyroid hormone, which helps to regulate calcium metabolism).
Bisphosphonates such as alendronate and risedronate bind permanently to the surface of bone and interfere with osteoclast activity. This allows the osteoblasts to outpace the rate of resorption. The most common side effects are nausea, abdominal pain and loose bowel movements. However, alendronate is reported to also cause irritation and inflammation of the esophagus, and in some cases, ulcers of the esophagus. Risedronate is chemically different from alendronate and has less likelihood of causing esophagus irritation. However, certain foods, calcium, iron supplements, vitamins and minerals, or antacids containing calcium, magnesium, or aluminum can reduce the absorption of risedronate, thereby resulting in loss of effectiveness.
The most common side effect of Raloxifen and other SERMS (such as Tamoxifen) are hot flashes. However, Raloxifene and other hormone replacement therapies have been shown to increase the risk of blood clots, including deep vein thrombosis and pulmonary embolism, cardiovascular disease and cancer.
Calcitonin is not as effective in increasing bone density and strengthening bone as estrogen and the other anti-resorptive agents. Common side effects of either injected or nasal spray calcitonin are nausea and flushing. Patients can develop nasal irritations, a runny nose, or nosebleeds. Injectable calcitonin can cause local skin redness at the site of injection, skin rash, and flushing.
A situation demonstrative of the link between several disorders or disease states involving bone remodelling is that of the use of etidronate (Didronel) first approved by the FDA to treat Paget's disease. Paget's disease is a bone disease characterized by a disorderly and accelerated remodelling of the bone, leading to bone weakness and pain. Didronel has been used ‘off-label’ and in some studies shown to increase bone density in postmenopausal women with established osteoporosis. It has also been found effective in preventing bone loss in patients requiring long-term steroid medications (such as Prednisone or Cortisone). However, high dose or continuous use of Didronel can cause another bone disease called osteomalacia. Like osteoporosis, osteomalacia can lead to weak bones with increased risk of fractures. Because of osteomalacia concerns and lack of enough studies yet regarding reduction in the rate of bone fractures, the United States FDA has not approved Didronel for the treatment of osteoporosis.
Osteoporosis therapy has been largely focused on antiresorptive drugs that reduce the rate of bone loss but emerging therapies show promise in increasing bone mineral density instead of merely maintaining it or slowing its deterioration. The osteoporosis early stage pipeline consists largely of drug candidates in new therapeutic classes, in particular cathepsin K inhibitors, osteoprotegerin and calcilytics as well as novel bisphosphonates. Some of these are examples where novel drugs exploiting genomics programs are being developed based on a deeper understanding of bone biology and have the potential to change the face of treatment of bone disorders in the long term.
The present invention describes the use of antibodies specific for Siglec-15 for the diagnosis, prognosis, and treatment (including prevention) of cancer or bone loss (e.g., severe or excessive bone loss associated with bone-related disease or associated with an increase in osteoclast differentiation or activity). In particular, the present invention relates to the use of anti-Siglec-15 antibodies for inhibiting the differentiation of osteoclasts.
Sialic-acid-binding immunoglobulin-like lectins (Siglecs) are members of the immunoglobulin (Ig) superfamily that have the ability to interact with sialic acids (McMillan and Crocker, 2008; Crocker et al., 2007). There are several Siglec family members that all share specific structural features, in particular, displaying an amino-terminal V-set Ig domain that binds to sialic acid and a variable number of C2-set Ig domains. These membrane receptors are generally expressed in highly specific manners and many of the family members are expressed in hematopoietic cells (McMillan and Crocker, 2008). These proteins are thought to promote cell-cell interactions, mediate signalling, and regulate immune functions through the recognition of glycans (Crocker et al., 2007). Sialic acids are nine-carbon sugars typically located at the ends of complex glycoconjugates on the surface of cells. They can be attached to a wide variety of proteins and lipids (McMillan and Crocker, 2008).
Siglec-15 is one of the most recently described Siglec family members that have a high homology to Siglec-14 (Angata et al., 2007). These authors reported that it preferentially binds to sialyl Tn structure and that it interacts with DAP12 and DAP10. The functional significance of these interactions is not known but it was proposed that Siglec-15 probably harbors an activating function (Angata et al., 2007). Despite these preliminary insights into a potential role in mammals of Siglec-15, important advances in the understanding of the biological function of the protein were contributed when the sequence was identified as part of a screen to discover novel regulators of osteoclast differentiation (Sooknanan et al. 2007). In this patent application, it was revealed that attenuation of the Siglec-15 transcript by RNA interference in a mouse model of osteoclastogenesis resulted in significant reduction of differentiation of precursors in response to RANKL treatment. Similar results were disclosed in human osteoclasts. Furthermore, the studies presented in this disclosure also showed that the localization of Siglec-15 at the cell membrane was necessary for its function in osteoclast differentiation. Furthermore, a recent publication showed that the presence of sialic acid at the end of surface glycoconjugates was required for proper osteoclast differentiation and were probably important for the fusion of osteoclast precursor cells (Takahata et al., 2007). This last observation creates a direct functional link between sialic acid binding and the expression of Siglec-15 in differentiating osteoclasts and strongly suggested that Siglec-15 plays a role in the early differentiation program of osteoclast precursors.
Thus, the expression profile of Siglec-15, its strong inducibility during osteoclast differentiation, its localization at the surface of the membrane, and its structural features all contribute to the feasibility of targeting this protein at the cell surface with monoclonal antibodies. The only other example of monoclonal antibody-based therapy that target osteoclasts is denosumab, a human monoclonal antibody that is specific for RANKL (Ellis et al. 2008). The present invention relates to the use of anti-Siglec-15 antibodies or antigen binding fragments as blockers of osteoclast differentiation in the detection or treatment of bone loss, especially in the context of bone-related diseases or in the context of increased osteoclast differentiation or activity. The present invention also relates to the use of antibodies or antigen binding fragments in the detection or treatment of cancer.